1. Field of the Invention
The present invention relates to digital graphics systems. More specifically, the present invention relates to methods and circuits for sampling analog video signals for digital display systems.
2. Discussion of Related Art
Analog video displays such as cathode ray tubes (CRTs) dominate the video display market. Thus, most electronic devices that require video displays, such as computers and digital video disk players, output analog video signals. As is well known in the art, an analog video display sequentially reproduces a large number of still images to give the illusion of full motion video. Each still image is known as a frame. For television, 60 frames are displayed in one second. For computer applications, the number of frames per seconds is variable with typical values ranging from 56 to 100 frames per seconds.
FIG. 1(a) illustrates a typical analog video display 100. Analog video display 100 comprises a raster scan unit 110 and a screen 120. Raster scan unit 110 generates an electron beam 111 in accordance with an analog video signal VS, and directs electron beam 111 against screen 120 in the form of sequentially-produced horizontal scanlines 101-109, which collectively form one frame. Screen 120 is provided with a phosphorescent material that is illuminated in accordance with the video signal VS transmitted in electron beam 111 to produce contrasting bright and dark regions that create an image, such as the diamond shape shown in FIG. 1(a). After drawing each scanline 101-108, raster scan unit 110 performs a horizontal flyback 130 to the left side of screen 120 before beginning a subsequent scanline. Similarly, after drawing the last scanline 109 of each frame, raster scan unit 110 performs a vertical flyback 131 to the top left corner of screen 120 before beginning a subsequent frame. To avoid generating an unwanted flyback traces (lines) on screen 120 during horizontal flyback 130, video signal 130 includes a horizontal blanking pulse that turn off electron beam 111 during horizontal flyback 130. Similarly, during vertical flyback 135, video signal VS includes a vertical blanking pulse that turns off electron beam 111 during vertical flyback 135.
FIG. 1(b) illustrates a typical analog video signal VS for analog video display 100. Video signal VS is accompanied by a horizontal synchronization signal HSYNCH and a vertical synchronization signal VSYNCH (not shown). Vertical synchronization signal VSYNCH contains vertical synch marks to indicate the beginning of each new frame. Typically, vertical synchronization signal VSYNCH is logic high and each vertical synch mark is a logic low pulse. Horizontal synchronization signal HSYNCH contains horizontal synch marks (logic low pulses) 133, 134, and 135 to indicate the beginning of data for a new scanline. Specifically, horizontal synch mark 133 indicates video signal VS contains data for scanline 103; horizontal synch mark 134 indicates video signal VS now contains data for scanline 104; and horizontal synch mark 135 indicates video signal VS now contains data for scanline 105.
Video signal VS comprises data portions 112, 113, 114, and 115 that correspond to scanlines 102, 103, 104, and 105, respectively. Video signal VS also comprises horizontal blanking pulses 123, 124 and 125, each of which is located between two data portions. As explained above, horizontal blanking pulses 123, 124, and 125 prevent the electron beam from drawing unwanted flyback traces on analog video display 100. Each horizontal blanking pulse comprises a front porch FP, which precedes a horizontal synch mark, and a back porch BP which follows the horizontal synch mark. Thus, the actual video data for each row in video signal VS lies between the back porch of a first horizontal blanking pulse and the front porch of the next horizontal blanking pulse.
Digital video display units, such as liquid crystal displays (LCDs), are becoming competitive with analog video displays. Typically, digital video display units are much thinner and lighter than comparable analog video displays. Thus, for many video display functions, digital video displays are preferable to analog video displays. For example, a 19 inch (measured diagonally) analog video display, which has a 17 inch viewable area, may have a thickness of 19 inches and weigh 80 pounds. However, a 17 inch digital video display, which is equivalent to a 19 inch analog video display, may be only 4 inches thick and weigh less than 15 lbs. However, most computer systems are designed for use with analog video displays. Most computer systems output analog video signals, such as video signal VS and horizontal synchronization signal HSYNCH. Thus, the analog video signal provided by a computer must be converted into a format compatible with digital display systems.
FIG. 1(c) illustrates a typical digital display 150. Digital display 150 comprises a grid of picture elements ("pixels") divided into rows 151-159 and columns 161-174. Each data portion (e.g. data portions 112, 113, 114, and 115) is treated as one row of a digital display. Each data portion is also divided into smaller portions and digitized to form pixel data that is transmitted to its designated pixel using row driver 180 and column driver 190. For most computer applications, the number of columns can be determined by the vertical resolution, which is equal to the number of rows. For example, common computer display formats include 640 columns by 480 rows (640.times.480), 800 columns by 600 rows (800.times.600), 1024 columns by 768 rows (1024.times.768), and 1280 columns by 1024 rows (1280.times.768). If video signal VS (FIG. 1(b)) contains 480 rows, then data portion 114 is divided into 640 smaller portions, which are individually digitized to form 640 pixel data for pixels of one row. Typically, the digitized image is stored in a frame buffer, which is used to drive row driver 180 and column driver 190. The actual physical digital display unit may contain thousands of pixels, thus the digital image stored in the frame buffer must be scaled accordingly before being displayed on the digital display.
To create a digital display from an analog video signal, the analog video signal must be digitized at precise locations to form the pixels of a digital display. Typically, a sampling clock signal is used to digitize video signal VS. However, the sampling clock signal must have a frequency and phase such that the sampling clock has the same number of periods during a data portion of video signal VS as the number of pixels to be sampled in that data portion. Creation of the sampling clock signal is complicated because the size of the front porch and back porch of a video signal may differ from computer to computer. Furthermore, different display resolutions on the same computer may also use differently sized front porches and back porches. Hence, there is a need for a method or circuit to generate a sampling clock signal that can be used to convert analog video signals into digital display data.